The present disclosure relates to an image forming apparatus including a development device supplying a toner to an image carrier.
Conventionally, in an image forming apparatus of a photographic manner, such as a copying machine or a printer, a development device supplying a toner to an image carrier by employing a high voltage is provided in order to form a toner image on the image carrier, such as a photosensitive drum, on which an electrostatic latent image on the basis of image data is formed.
The development device includes a developing roller having a rotating shaft parallel to the image carrier. The development device applies to the developing roller a high voltage obtained by superimposing an alternating current voltage and a direct current voltage on each other by a high voltage board, causes the developing roller to carry a charged toner, and electrically supplies the charged toner to the image carrier in a location in which the image carrier and the developing roller are close to each other. However, a surface of the developing roller is not always in complete parallel to a surface of the image carrier, and may have a runout of dozens of μm in size. Therefore, a close distance between the image carrier and the developing roller is varied in a rotation cycle of the developing roller. Even if a high voltage applied to the developing roller is stabilized, when the close distance between the image carrier and the developing roller is varied, an electric field between the image carrier and the developing roller is varied, and thereby, a toner supply amount is varied. For example, if the close distance between the image carrier and the developing roller is decreased, the toner supply amount is increased. On the other hand, if the close distance is increased, the toner supply amount is decreased. Thus, if the toner supply amount to the image carrier is varied in one rotation cycle of the developing roller, an uneven density in each rotation cycle of the developing roller may occur in a printed image.
In order to restrain this uneven density, direct current voltage correction is known to vary a direct current voltage in accordance with variation in the close distance between the image carrier and the developing roller. Specifically, if the close distance between the image carrier and the developing roller is varied, an alternating current between the image carrier and the developing roller is varied, and therefore, the alternating current is sensed by a high voltage board applying a high voltage to the developing roller and the direct current voltage correction is carried out to vary a direct current voltage in accordance with the variation of the alternating current. For example, by the direct current voltage correction, in a case where the alternating current is increased, the direct current voltage is lowered and, in a case where the alternating current is decreased, the direct current voltage is heightened, and thereby, the electric field between the image carrier and the developing roller is maintained constant.
For example, the development device detects a current of an alternating current component from a vibration bias voltage applied between the image carrier, such as the photosensitive drum, and a developer carrier, such as a developing sleeve, and varies a voltage of a direct current component in accordance with the detected value each time.
In addition, the image forming apparatus may include an electric discharge sensing circuit in order to measure an optimal alternating current voltage to be superimposed on a development bias of the development device. The electric discharge sensing circuit senses air discharge occurring between the image carrier and the developing roller. The development device is controlled in a cycle of measuring the optimal alternating current voltage so as to fix the direct current voltage to a constant value and to gradually heighten the alternating current voltage. The development device then applies the superimposed voltage as mentioned above to the developing roller while gradually heightening the alternating current. At this time, because the electric discharge sensing circuit senses the electric discharge between the image carrier and the developing roller, the optimal alternating current voltage is measured on the basis of the alternating current voltage when the electric discharge is sensed.
Alternatively, for example, the image forming apparatus may have the photosensitive drum, the developing roller arranged with a gap with respect to the photosensitive drum and connected to an alternating current voltage applying part, a detecting part detecting an occurrence of electric discharge between the developing roller and the photosensitive drum, and a controller controlling each component of the apparatus and recognizing an occurrence of electric discharge when an output of the detecting part is inputted.
However, in the case of correcting the direct current voltage constituting the high voltage as mentioned above, because the alternating current is increased at the time of rise of the alternating current voltage, the direct current voltage may be lowered. Therefore, the direct current voltage cannot be risen until the alternating current voltage is stabilized at a target voltage and the alternating current is stabilized, and as a result, the rise of the direct current voltage at the time of a printing start is delayed. Thus, in a cycle in which the direct current voltage does not rise, because a sufficient high voltage cannot be applied to the developing roller, the toner of a sufficient amount cannot be supplied to the image carrier, and therefore, an image having a thin toner at a tip end is printed.
Moreover, the development device may be composed of a touchdown development manner to include the developing roller supplying the toner to the image carrier, and a magnet roller supplying the toner to the developing roller. The development device of such a touchdown development manner has, as operating modes, a normal printing mode and a toner removing mode (a cleaning mode) collecting an old toner on the developing roller to the magnet roller at a timing between sheets. In this cleaning mode, in order to remove the toner from the developing roller, the high voltage (the alternating current voltage) applied to the developing roller is lowered. Therefore, when the cleaning mode is switched to the printing mode at a timing between sheets in continuous printing, there is a need to heighten the alternating current voltage again. However, as mentioned above, because the direct current voltage cannot be risen until the alternating current is stabilized, on an image printed immediately after switching to the printing mode, the toner at the tip end part becomes thin.
In addition, in the image forming apparatus including the electric discharge sensing circuit as mentioned above, in order to measure the optimal alternating current voltage to be superimposed on the development bias, the development device applies the superimposed voltage to the developing roller while gradually heightening the alternating current voltage. However, if the alternating current voltage is heightened, the direct current voltage is controlled so as to be lowered by the direct current voltage correction as mentioned above. It is feared that the electric discharge sensing circuit mistakenly senses such variation of the direct current voltage as air discharge, in spite of the fact that such air discharge does not occur between the image carrier and the developing roller.